This application is related to and claims priority from Japanese Patent Applications No. 2001-126516 filed on Apr. 24, 2001 and No. 2001-130126 filed on Apr. 26, 2001, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates to a controller of an electrical load such as an electrical heater driven by direct-current electrical power. The controller is suitably used for a vehicle air conditioner.
2. Description of Related Art
In a vehicle, having an electrical drive motor for a vehicle running, such as a hybrid vehicle and a fuel-cell electrical vehicle, electrical power is supplied to the electrical drive motor from a direct-current (DC) power source having a supply voltage of approximately 300 volts (V). For example, in an air conditioner for this vehicle described in JP-A-10-157446, an electrical heater (electrical load) is used for heating air to be blown into a passenger compartment, and a high voltage of approximately 300V is applied to the electrical heater from the DC power source. Here, a thermal fuse can be used in order to protect the electrical heater by disconnecting a current-carrying circuit at an abnormal heating of the electrical heater. Although reliability of the thermal fuse is high in an alternating current (AC), reliability thereof is known to be low in a DC, especially in a high-voltage DC. Therefore, it is difficult to use the thermal fuse, for protecting the electrical load driven in the high-voltage DC.
On the other hand, in a lamp regulator described in JP-A-9-69392, DC output is converted to rectangular-wave AC output as shown in FIG. 10, and the rectangular-wave AC output is supplied to a lamp (electrical load), thereby improving the reliability of the thermal fuse. However, since only a polarity is inverted in the rectangular-wave AC output shown in FIG. 10, a voltage is always applied to the thermal fuse while the AC output is applied to the lamp. Accordingly, an arc, generated when the thermal fuse is blown, does not readily disappear, and the thermal fuse may be welded again after being arced.
In view of the foregoing problem, it is an object of the present invention to provide an electrical load controller which can accurately control an operation of a thermal fuse for an electrical load driven by DC power.
In is an another object of the present invention to provide a vehicle air conditioner using the electrical load controller.
According to the present invention, in an electrical load controller, electrical power is supplied to the electrical load from a direct-current (DC) power source through an electrical circuit, and a thermal fuse is blown at an abnormal heating of the electrical load so that the electrical circuit is disconnected at the abnormal heating. A controller controls a work amount of the electrical load by intermittently controlling electrical power to be supplied to the electrical load. In the electrical load controller, the controller intermittently stops the power supply to the electrical load even in a maximum work area where the controller controls the work amount of the electrical load at a maximum work amount. Since the power supply to the electrical load is intermittently stopped even when the electrical load is operated by the maximum work amount, an arc, generated when the thermal fuse is blown, disappears while the power supply is stopped. Therefore, it can prevent the thermal fuse from being again welded after being blown. Thus, the thermal fuse can be accurately operated while protecting the electrical load driven by DC power.
When the controller duty-controls the operation of the electrical load by the maximum work amount, the duty ratio is set lower than 100%.
Preferably, the electrical load has a plurality of electrical load parts connected in parallel with respect to the DC power source, each of the electrical load parts is energized by the DC power source through each circuit part of the electrical circuit, the thermal fuse has a plurality fuse parts each of which is connected to each of the electrical loads in series, and each fuse part is blown at abnormal heating of each electrical load part to disconnect each circuit part. Therefore, each fuse part of the thermal fuses can be used in a condition where high reliability can be obtained, and the fuse parts can be actually operated while effectively protecting the load parts driven with DC power having a high voltage and a high current. Furthermore, each load part of the electrical load can be effectively protected using the fuse parts, without using an additional relay, an additional temperature sensor and the like, thereby reducing production cost.
Accordingly, when the electrical load controller is used for a vehicle air conditioner so that the electrical load is used as an electrical heater for heating air blown into a passenger compartment, heating operation in the vehicle air conditioner can be accurately controlled.